1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a zoom lens system for use in an image projecting apparatus such as a microfilm reader or a microfilm reader/printer which projects microfilm images and has a limited conjugate length.
2. Description of the Prior Art
A microfilm can include a plurality of images thereon which are not always recorded in the same direction. Therefore, an image projecting apparatus such as a microfilm reader or a microfilm reader/printer has a prism disposed between a projecting lens and an image receiving plane for optically rotating images. This prism normally is disposed adjacent an enlargement side of the projecting lens, and therefore the space for accommodating the projecting lens is limited by the prism and the microfilm. Thus, this arrangement is effective to shorten the entire length of the projecting lens for compactness of the device.
Shortening the entire length of the projecting lens, however, sets a great limitation to the designing of a lens of about 10 magnifications having a long focal length. It sets an even greater limitation to a zoom lens system having a magnification range on the order of 10-power.
Zoom lens systems used as a projecting lens system of an image projecting apparatus are disclosed in U.S. Pat. No. 4,746,204, U.S. Pat. No. 4,743,102 and U.S. Pat. No. 4,750,820.
The zoom lens systems disclosed in these patents are negative-positive, two group lens systems comprising a negative lens unit at an enlargement side and a positive lens unit at a reduction side. Such a negative-positive lens system has a wide angle of view but, because of its construction, has a principal point located relatively close to the reduction side, and hence the disadvantage of having an increased overall length of the lens system.